The present invention relates generally to methods and compositions for treating subterranean formations penetrated by a well bore, and more particularly, but not by way of limitation, to methods and compositions for temporarily plugging portions of formations penetrated by a well bore by injecting into the formations through the tubing or casing a self-thickening and thinning fluid composition.
A problem encountered in treating operations, wherein temporary plugging or blocking of a portion of a well bore is to be effected utilizing self-thickening fluid compositions is stability of the plugging gel to heat. By stability to heat is meant the retention of the increased or greater viscosity properties under the conditions of use. Such compositions to be satisfactory should be sufficiently stable to resist degeneration by the heat of the formation for a period of time sufficient to accomplish the intended purpose, that is, plugging or decreasing formation permeability for a desired period of time. The degree of stability required in any particular operation will vary with such operating variables as the type of formation being treated, the temperature of the formation, the well depth (time to pump the fluid composition down the well and into the formation), the polymer concentration of the composition, and the like.
The temperature of the formation usually has a pronounced effect on the stability of the gelled fluid compositions and, generally speaking, is one of the most important operating variables when considering stability. Increased formation temperatures usually have at least one undesirable effect. Such an effect can be degeneration of the compositions, that is, a decrease in viscosity. Thus, some compositions which would be satisfactory plugging agents in a low temperature formation might not be satisfactory in formations having higher temperatures, such as are encountered in deeper wells.
A problem which also is encountered when using thickened compositions in treating formations is the ease of removal of the treating composition after the operation is completed. Some thickened or highly viscous solutions are difficult to remove from the pores of the formation after the operation is complete. Sometimes a clogging residue can be left in the pores of the formation. This can inhibit the production of fluids from the formation and can require costly cleanup operations. It would be desirable to have gelled fluid compositions which break down to a lesser viscosity within a short time after the operation is completed.
Presently, guar gum and guar gum derivatives along with other natural gums are utilized in the preparation of viscous fluids utilized in the treatment of subterranean formations to provide adequate viscosity to the fluid when utilized at temperatures in excess of about 125.degree. F. to about 350.degree. F. Guar gum and the guar derivatives have the capability of being crosslinked by a variety of polyvalent metals, such as, for example, titanium, zirconium, chromium, antimony, niobium, and the like. While not entirely satisfactory, these viscosifiers form a "gelled" fluid having a viscosity greater than the uncrosslinked guar gum and derivatives. However, guar and the other gums typically contain substantial amounts, that is, from about 1.5 to in excess of 10 percent by weight of insoluble matter. The presence of such insoluble matter in a fluid is highly undesirable when the temporary blocking agent thins since it may clog the pores of the formation or the fracture.
It would appear that cellulose ethers, such as hydroxyethylcellulose, would be a viable alternative to guar gum, guar derivatives or other gums for use in gelling fluids because of its substantially lower insoluble matter content. However, such has not been the case because the cellulose ethers have generally been unable to provide gelled fluids having the necessary viscosity at elevated temperatures which can be achieved through the use of guar gum and guar derivatives. Further, hydroxyethylcellulose and its derivatives generally have not been capable of adequate crosslinking, which is a necessary prerequisite to provide sufficient viscosity levels to the fluids for more severe applications.
The present invention provides a solution for, or at least mitigates, the above-discussed problems. The present invention provides improved methods for treating subterranean formations to temporarily reduce the permeability of the formation utilizing a novel gelling agent.